Sev­er­al phys­i­cists say they’ve con­firmed strange pre­dic­tions of mod­ern phys­ics that clash with our most bas­ic no­tions of real­i­ty and even sug­gest—some sci­en­tists and phi­loso­phers say—that real­i­ty is­n’t there when we’re not look­ing.

The pre­dic­tions have lurked with­in quan­tum me­chan­ics, the sci­ence of the small­est things, since
the field emerged in the 1920s; but not all phys­i­cists ac­cept­ed them. They were
un­dis­put­edly con­sist­ent with ex­pe­ri­ments, but ex­pe­ri­ments might not re­veal eve­ry­thing.

New tests—de­signed more specif­i­cally than be­fore to probe the real­i­ty ques­tion—have yielded un­set­tling re­sults, say re­search­ers who pub­lished
the find­ings in the April 19 is­sue of the re­search jour­nal Na­ture. One of their col­leagues called the find­ings in­tri­guing but in­con­clu­sive.

A wave with a sim­ple, or
lin­e­ar, po­lar­i­za­tion is tilted in one di­rec­tion as above. El­lip­ti­cal
po­lar­i­za­tion means the an­gle of po­lar­i­za­tion changes con­stant­ly as the
wave moves, as the corkscrew-like im­age be­low.

The background

Quan­tum phys­i­cists have long not­ed that sub­a­tom­ic par­t­i­cles seem to move ran­dom­ly. For in­stance, one can meas­ure a par­t­i­cle’s lo­ca­tion at a giv­en mo­ment, but can’t know ex­act­ly where it would be just be­fore or af­ter.

Phys­i­cists de­ter­mined that the ran­dom­ness was­n’t just an ap­pear­ance due to our ig­no­rance of the de­tails of the mo­tion, but an in­es­cap­a­ble prop­er­ty of the par­t­i­cles them­selves.

Rath­er per­sua­sive ev­i­dence for this lay in math. Par­t­i­cles, for rea­sons no one quite knows, some­times act like waves. When they come to­geth­er, they cre­ate the same types of com­plex pat­terns that ap­pear when wa­ter rip­ples from dif­fer­ent di­rec­tions over­lap.

But a par­t­i­cle, be­ing at least some­what con­fined in space, nor­mal­ly acts on­ly as a small “wave pack­et”—a clus­ter of a few rip­ples in suc­ces­sion—un­like fa­mil­iar waves, in which doz­ens or thou­sands pa­rade along.

It turns out there is a math­e­mat­i­cal way to rep­re­sent a wave pack­et; but you must start by rep­re­senting an in­fi­nite­ly re­peat­ing wave, which is a sim­pler for­mu­la. Adding up many such de­pic­tions, if you choose them prop­er­ly, gives the packet.

Yet there’s a catch: each of these com­po­nents must have a slight­ly dif­fer­ent wave speed. Thus, the com­plete pack­et has no clear-cut speed. Nor, con­se­quent­ly, does the par­t­i­cle.

The previous experiments

Pre­cise­ly in line with such math, ex­pe­ri­ments find that par­t­i­cle speed is some­what ran­dom, though the ran­dom­ness fol­lows rules that again mir­ror the equa­tions. When you meas­ure speed, you do get a num­ber, but that won’t tell you the speed a mo­ment be­fore or af­ter. In es­sence, phys­i­cists con­clud­ed, the par­t­i­cle has no de­fined ve­loc­i­ty un­til you meas­ure it. Si­m­i­lar con­sid­er­a­tions turned out to hold for its lo­ca­tion, spin and oth­er prop­er­ties.

The im­pli­ca­tions were huge: the ran­dom­ness im­plied that key prop­er­ties of these ob­jects, per­haps the ob­jects them­selves, might not ex­ist un­less we are watch­ing. “No el­e­men­ta­ry phe­nom­e­non is a phe­nom­e­non un­til it is an ob­served phe­nom­e­non,” the cel­e­brat­ed Prince­ton Uni­ver­si­ty phys­i­cist John Wheel­er put it.

Still, human-made math­e­mat­i­cal mod­els don’t nec­es­sar­i­ly re­flect ul­ti­mate truth, even if they do match ex­pe­ri­men­tal re­sults bril­liant­ly. And those tests them­selves might miss some­thing. Sci­en­tists in­clud­ing Ein­stein
balked at the ran­dom­ness idea—“God does not play dice,” he fa­mous­ly fumed—and the con­se­quent col­lapse of cher­ished as­sump­tions. The great phys­i­cist joined oth­ers in pro­pos­ing that there ex­ist some yet-unknown fac­tors, or “hid­den vari­ables,” that in­flu­ence par­t­i­cle prop­er­ties, mak­ing these look ran­dom with­out tru­ly be­ing so.

Phys­i­cists in due course de­signed ex­pe­ri­ments to test for hid­den vari­ables. In 1964 John Bell de­vised such a test. He ex­ploited a cu­ri­ous phe­nom­e­non called “en­tan­gle­ment,” in which know­ing some­thing about one par­t­i­cle some­times tells you a cor­re­spond­ing prop­er­ty of anoth­er, no mat­ter the dis­tance be­tween them.

An ex­am­ple oc­curs when cer­tain par­t­i­cles de­cay, or break up, in­to two pho­tons—par­t­i­cles of light. These fly off in op­po­site di­rec­tions and have the same po­lar­i­za­tion, or amount by which the wave is tilted in space. De­tec­tors called po­lar­iz­ers can meas­ure this at­trib­ute. Po­lar­iz­ers are like ti­ny fences with slits. If the slits are tilted the same way as the wave, it goes through; if op­po­sitely, it does­n’t; if some­where in be­tween, it may or may not pass.

If you meas­ure the two op­po­sitely-flying pho­tons with po­lar­iz­ers tilted the same way, you get the same re­sult for both. But if one of the po­lar­iz­ers is tilted a bit, you will get oc­ca­sion­al dis­a­gree­ments be­tween the re­sults.

What if you al­so tilt the sec­ond po­lar­izer by the same amount, but the op­po­site way? You might get twice as many dis­a­gree­ments, Bell rea­soned. But you might al­so get less than that, be­cause some po­ten­tial dis­agree­ments could can­cel each oth­er out. For ex­am­ple: two pho­tons might be blocked where­as orig­i­nal­ly they both would have pas­sed, so two de­vi­a­tions from the orig­i­nal re­sult lead to an agree­ment.

All this fol­lows from log­ic. It al­so de­pends on cer­tain rea­son­a­ble as­sump­tions, in­clud­ing that the par­t­i­cles have a real po­lar­i­za­tion
wheth­er it’s meas­ured or not.

But Bell, in an ar­gu­ment known as Bell’s The­o­rem, showed that quan­tum me­chan­ics pre­dicts anoth­er out­come, im­ply­ing this “real­i­ty” as­sump­tion might be wrong. Quan­tum me­chan­ics claims that the num­ber of dis­a­gree­ments be­tween the re­sults when both po­lar­iz­ers are op­po­sitely tilt­ed—com­pared to one be­ing tilt­ed—can be more than twice as many. And ex­pe­ri­ments have borne this out.

The rea­sons why have to do with yet anoth­er odd pre­dic­tion of quan­tum me­chan­ics. Once you de­tect the pho­ton as ei­ther hav­ing crossed the po­lar­izer or not, then it’s ei­ther po­lar­ized ex­act­ly in the di­rec­tion of the in­stru­ment, or the op­po­site way, re­spec­tive­ly. It can’t be po­lar­ized at any oth­er an­gle. And its “twin” must be iden­ti­cal­ly po­lar­ized. All this puts ad­di­tion­al con­s­t­raints on the sys­tem such that the num­ber of dis­a­gree­ments can rise com­pared to the “log­ical” re­sult.

Past ex­pe­ri­ments have con­firmed the seem­ingly non­sen­si­cal out­come. Yet this alone this does­n’t dis­prove the “real­i­ty” hy­poth­e­sis, re­search­ers say. There’s one oth­er pos­si­bil­i­ty, which is that the par­t­i­cles are some­how in­stan­ta­ne­ously com­mu­ni­cat­ing, like telepaths.

The new experiment

The new ex­pe­ri­ment was de­signed to side­step this loop­hole: it was set up so that even al­low­ing for in­stan­ta­ne­ous com­mu­ni­ca­tion could­n’t ex­plain the “non­sen­si­cal” out­come, at least not eas­i­ly. One would al­so have to drop the no­tion that pho­tons have a def­i­nite po­lar­i­za­tion in­de­pend­ent of any meas­urement.

The work, by Si­mon Groe­blacher and col­leagues at the Aus­tri­an Acad­e­my of Sci­ences’ In­sti­tute for Quan­tum Op­tics and Quan­tum In­for­ma­tion in Vi­en­na, was based not on Bell’s The­o­rem, but on a re­lat­ed the­o­rem more re­cent­ly de­vel­oped by An­tho­ny
Leg­gett at the Uni­ver­si­ty of Il­li­nois at Urbana-Champaign.

Full ex­pe­ri­ments based on Leggett’s con­cept re­quired an­a­lyz­ing pho­ton-waves that are po­lar­ized “el­lip­ti­cally,” which means
a wave’s tilt changes con­stant­ly. One can de­tect this by sup­ple­ment­ing the po­lar­izer with a strip of ma­te­ri­al that’s bi­re­frin­gent, mean­ing it bends light dif­fer­ently de­pend­ing on its di­rec­tion.

The re­sults in­deed dis­proved that pho­tons have a def­i­nite, in­de­pend­ently ex­isting po­lar­i­za­tion, Markus As­pelmeyer, a mem­ber of the re­search team, wrote in an e­mail. The find­ings thus spell trou­ble for one “plau­si­ble no­tion of
real­ism,” he added, though oth­ers could con­ceiv­a­bly sur­vive.

Not eve­ry­one is con­vinced. “The con­clu­sion one draws is more a ques­tion of taste than log­ic,” wrote Alain As­pect, who con­ducted the first con­clu­sive tests of Bell’s The­o­rem, in a com­men­tary in the same is­sue of the jour­nal. As­pect, of the École Poly­tech­nique in
Pa­lai­s­eau, France, ar­gued that the find­ings can still be ex­plained by claim­ing cer­tain forms of in­s­tan­t­a­ne­ous com­mu­ni­ca­tion. But he con­ced­ed that he too is in­clined to re­nounce as­pects of
real­ism in­stead. Such ex­pe­ri­ments, and the re­sulting de­bates, “al­low us to look deeper in­to the great mys­ter­ies of quan­tum me­chan­ics,” he added.
* * *

Several physicists say they’ve confirmed strange predictions of modern physics that clash with our most basic notions of reality and even suggest—some scientists and philosophers say—that reality isn’t there when we’re not looking.
The predictions have lurked within quantum mechanics, the science of the smallest things, since its birth in the 1920s; but not all physicists accepted them. They were undisputedly consistent with experi ments, but experi ments might not reveal everything.
New tests—designed more specifically than before to probe the reality question—have yielded unsettling results, say the researchers, who published their work in the April 19 issue of the research journal Nature. One of their colleagues called the findings intriguing but inconclusive.
Quantum physicists have long noted that subatomic particles seem to move randomly. For instance, one can measure a particle’s location at a given moment, but can’t know exactly where it would be just before or after. Physicists determined that the randomness wasn’t just an appearance due to our ignorance of the details of the motion, but an inescapable property of the particles themselves.
Rather persuasive evidence for this lay in math. Particles, for reasons no one quite knows, sometimes act like waves. When they come together, they create the same types of complex patterns that appear when water ripples from different directions overlap. But a particle, being at least somewhat confined in space, normally acts only as a small “wave packet”—a cluster of a few ripples in succession—unlike familiar waves, in which dozens or thousands parade along.
It turns out there is a mathematical way to represent a wave packet; but you must start by representing an infinitely repeating wave, which is a simpler formula. Adding up many such depictions, if you choose them properly, gives the packet. Yet there’s a catch: each of these components must have a slightly different wave speed. Thus, the complete packet has no clear-cut speed. Nor, consequently, does the particle.
Precisely in line with such math, experi ments find that particle speed is somewhat random, though the randomness follows rules that again mirror the equations. When you measure speed, you do get a number, but that won’t tell you the speed a moment before or after. In essence, physicists concluded, the particle has no defined velocity until you measure it. Similar considerations turned out to hold for its location, spin and other properties.
The implications were huge: the randomness implied that key properties of these objects, perhaps the objects themselves, might not exist unless we are watching. “No elementary phenomenon is a phenomenon until it is an observed phenomenon,” the celebrated Princeton University physicist John Wheeler put it.
Still, human-made mathematical models don’t necessarily reflect ultimate truth, even if they do match experi mental results brilliantly. And those tests themselves might miss something. Scientists including Einstein recoiled at the randomness idea—”God does not play dice,” he famously fumed—and the consequent collapse of cherished assumptions. The great physicist joined others in proposing that there exist some yet-unknown factors, or “hidden variables,” that influence particle properties, making these just look random without truly being so.
Physicists in due course designed experi ments to test for hidden variables. In 1964 John Bell devised such a test. He exploited a curious phenomenon called “entanglement,” in which knowing something about one particle sometimes tells you a corresponding property of another, no matter the distance between them.
An example occurs when certain particles decay, or break up, into two photons—particles of light. These fly off in opposite directions and have the same polarization, or amount by which the wave is tilted in space. Detectors called polarizers can measure this attribute. Polarizers are like tiny fences with slits. If the slits are tilted the same way as the wave, it goes through; if oppositely, it doesn’t; if somewhere in between, it may or may not pass.
If you measure the two oppositely-flying photons with polarizers tilted the same way, you get the same result for both. But if one of the polarizers is tilted a bit, you will get occasional disagreements between the results.
What if you also tilt the second polarizer by the same amount, but the opposite way? You might get twice as many disagreements, Bell reasoned. But you might also get less than that, because some potential disagreements could cancel each other out. For example: two photons might be blocked whereas originally they both would have passed, so two deviations from the original result lead to an agreement.
All this follows from logic. It also depends on certain reasonable assumptions, including that the particles have a real polarization regardless of whether it’s measured or not.
But Bell, in an argument known as Bell’s Theorem, showed that quantum mechanics predicts another outcome, implying this “reality” assumption might be wrong. Quantum mechanics claims that the number of disagreements between the results when both polarizers are oppositely tilted—compared to one being tilted—can be more than twice as many. And experi ments have borne this out.
The reasons why have to do with yet another odd prediction of quantum mechanics. Once you detect the photon as either having crossed the polarizer or not, then it’s either polarized exactly in the direction of the instrument, or the opposite way, respectively. It can’t be polarized at any other angle. And its “twin” must be identically polarized. All this puts additional constraints on the system such that the number of disagreements can rise compared to the “logical” result.
Past experi ments have confirmed the seemingly nonsensical outcome. Yet this alone this doesn’t disprove the “reality” hypothesis, researchers say. There’s one other possibility, which is that the particles are somehow instantaneously com municating, like telepaths.
The new experi ment was designed to sidestep this loophole: it was set up so that even allowing for instantaneous com munication couldn’t explain the “nonsensical” outcome, at least not easily. One would also have to drop the notion that photons have a definite polarization independent of any measurement.
The work, by Simon Groeblacher and colleagues at the Austrian Academy of Sciences’ Institute for Quantum Optics and Quantum Information in Vienna, was based not on Bell’s Theorem, but on a related theorem more recently developed by Anthony Leggett at the University of Illinois at Urbana-Champaign.
Full experi ments based on his concept required analyzing a photon-waves that are polarized “elliptically,” which means the wave’s tilt changes constantly. One can detect this by supplementing the polarizer with a strip of material that’s birefringent, meaning it bends light differently depending on its direction.
The results indeed disproved that photons have a definite, independently existing polarization, Markus Aspelmeyer, a member of the research team, wrote in an email. The findings thus spell trouble for one “plausible notion of realism,” he added, though others could conceivably survive.
Not everyone is convinced. “The conclusion one draws is more a question of taste than logic,” wrote Alain Aspect, who conducted the first conclusive tests of Bell’s Theorem, in a commentary in the same issue of the journal. Aspect, of the École Polytechnique in Palaiseau, France, argued that the findings can still be explained by claiming certain forms of instantaneous com munication. But he conceded that he too is inclined to renounce aspects of realism instead. Such experi ments, and the resulting debates, “allow us to look deeper into the great mysteries of quantum mechanics,” he added.